(1) Field of the Invention
The present invention relates to a pneumatic tire having an improved chipping resistant property. A vulcanizate of a novel rubber composition comprising short fibers of syndiotactic (hereinafter abbreviated as syn)-1,2-polybutadiene is disposed in a tread portion of the tire.
(2) Description of the Prior Art
Generally, the tread portion of the tire is required to have a high wear resistance as a fundamental performance criteria. To this end, there have hitherto been proposed a method wherein carbon black having a high structure and a small particle size is compounded into a rubber composition for tread, a method wherein cis-1,4-polybutadiene rubber is used in the tread portion, and the like in order to improve the wear resistance of the tread portion. However, such use of carbon black or polybutadiene rubber has an undesirable tendency to promote a chipping phenomenon in the tread portion, resulting in not only serious deterioraton of tire appearance but also lowering of tire life. Particularly, such a chipping phenomenon appears remarkably on rough roads. Until now, a satisfactory means for preventing the occurrence of the chipping phenomenon has not been realized.
Moreover, attempts have been made to solve the above mentioned drawback by incorporating short fibers into the tread portion, but they have not yet been put to practical use. Because the short fiber reinforced rubber compositions as proposed in the prior art have drawbacks for example the adhesion of short fibers to rubber after the vulcanization is insufficient and hence a large creep is caused, and the life at fracture of rubber becomes considerably short when subjected to repeated extension.